Natural killer (NK) cell mediated cytolysis can be resolved into discrete stages of recognition, triggering, and lysis. Little is known concerning the molecular interactions involved in either of these three stages. We have isolated a monoclonal antibody (MAb), 3E12, which blocks recognition of K-562 target cells by NK effector cells. This reagent blocks the formation of effector cell-target cell conjugates, a necessary step required for the subsequent lysis of target cells. We propose to further characterize this structure and catalogue its appearance on putative targets cells in order to determine potential in vivo targets of NK cells. In addition to the development of this MAb, we have developed antiidiotypic antibody to 3E12 in an attempt to produce an antireceptor reagent which will react with the NK effector cell receptor. Several clones have been isolated which react specifically with 3E12 and which significantly alter the levels of NK cytotoxicity to K-562 target cells. This antireceptor reagent will be characterized and used to isolated and identify the NK cell receptor. We also propose additional studies on the mechanism of target cell lysis by NK effector cells. Special attention will be directed to mechanisms of nuclear damage that are seen during NK mediated cytolysis. Preliminary data has shown that nuclear damage is also a feature of cell damage mediated by soluble mediators of NK cytolysis. This will be further explored and attempts will be made to identify a nuclease that we have found in these soluble cytotoxic factors. This proposal will attempt to identify the source of this nuclease, effector cell or target cell, to determine if cytotoxicity consists of directed destruction of the target cell by factors from the effector cell or whether the target cell contributes to its own demise due to a signal from the effector cell which actives an internal disintegration message.